This invention relates to densified composite ceramic articles. More particularly, it is concerned with sintered, dense, polycrystalline silicon nitride articles having surface layers of a composition different from that of the interior body of the article.
Densified composite ceramic articles based upon silicon nitride have been the subject of considerable study. Powder mixtures predominantly comprising silicon nitride, when consolidated to densities approaching the theoretical maximum, result in the production of densified ceramic articles having desirable high temperature properties. These articles have refractory character, high temperature creep resistance, thermal shock resistance, and strength which make them ideal for many high temperature applications.
To produce densified composite ceramics from powder mixtures, three methods of consolidation generally have been employed: hot pressing, hot isostatic pressing, and so-called pressureless sintering.
Sintering is the preferred method of densifying composite ceramic articles because of both the cost and complexity of equipment needed for other methods and because of the difficulty of producing some intricately shaped ceramic articles by hot pressing or hot isostatic pressing.
Silicon nitride based ceramic articles having regions of differing density are known in the art and have generally been produced by methods which entail the bonding together of separate elements previously formed by hot pressing silicon nitride or by nitriding silicon preforms (see for example, U.S. Pat. Nos. 3,854,189; 3,885,294; 3,887,411; and 4,172,107).